Various electrophotographic processes are known as described in, for example, U.S. Pat. No. 2,297,691, JP-B-42-23910 and JP-B-24748 (the term "JP-B" as used herein means an "examined Japanese patent publication"). In general, an electric latent image is formed by various techniques using a photoconductive substance on a light-sensitive material, the latent image is developed with a toner, and then the resulting powder image is transferred to a substrate, such as paper, and fixed by heating or with solvent vapor, if desired, to obtain desired copies.
Known methods for visualizing an electric latent image with a toner include, for example, the magnetic brush method described in U.S. Pat. No. 2,874,063, the cascade developing method described in U.S. Pat. No. 2,618,552, and the powder cloud developing method described in U.S. Pat. No. 2,221,776.
A mixture of particles of a toner and a carrier is usually used as an electrophotographic developer. Typically, the toner is obtained by melt kneading a resin such as polystyrene, a styrene-butadiene copolymer or polyester, and a pigment or dye, such as carbon black or phtholocyanine blue, as colorant and then grinding the resulting kneaded product to 1 to 30 .mu.m. The carrier is a particle having an average particle diameter nearly equal to the particle diameter of the toner or up to 500 .mu.m formed of glass bead, iron, nickel or ferrite, or such a particle coated with various resins.
However, with a developer consisting of the above ingredients, the desired charging level (i.e. charging amount), charging speed, electric charge exchanging properties, uniformity of charging, dependency on circumstances of image quality, and durability of developer cannot be obtained. Thus, a charging controller additive has been added in an attempt to improve the developer.
The aforementioned requirements are not always satisfied, however, merely by adding a conventionally used charging controller. Particularly in a system in which a metal oxide powder having a electric resistance of 1.times.10.sup.2 to 1.times.10.sup.9 .OMEGA..multidot.cm is added as a charging controller, the desired charging level and electric charge exchanging properties do not correspond to the amount of the charging controller added. If the charging level is controlled to the desired level, the electric charge exchanging properties are markedly reduced, and attachment of the toner to non-image areas of a light-sensitive material is increased, and only copies having increased fog are obtained.